This invention relates to voltage regulators. More particularly, this invention relates to circuits and methods for providing multiple-input, single-output, low-dropout voltage regulators.
Multiple-input, single-output voltage regulators are widely used in applications such as uninterruptible power supplies where multiple input sources are used to provide continuous power to an associated circuit or device. These multiple input sources may be provided by utility-supplied DC voltage supplies, generators, or batteries, for example. In a typical uninterruptible power supply application, a multiple-input, single-output voltage regulator is connected to a utility-supplied DC voltage supply as a primary source of power and a battery as a secondary source of power. In other typical uninterruptible power supply applications, multiple-input, single-output voltage regulators are connected to one battery as a primary source of power and another battery as a secondary source of power. In all of these installations, when the primary source of power becomes inadequate or non-existent, the multiple-input, single-output voltage regulator detects this inadequacy and draws power from the secondary source of power instead of or in addition to the primary source of power.
In order to provide the maximum duration over which power can be supplied by uninterruptible power supplies that operate partially or entirely off battery power, many of these supplies incorporate low-dropout voltage regulators. The dropout voltage of a voltage regulator is the minimum additional voltage that must be provided at a voltage regulator's voltage supply input to maintain a regulated output voltage. Once this additional dropout voltage is not provided, the voltage regulator ceases to provide a regulated output and, thus, is said to enter "dropout." For example, a voltage regulator may only be able to provide a regulated output voltage of ten volts if it is supplied with an input voltage of at least twelve volts. In this example, the dropout voltage of the regulator is two volts. Because the voltage of a battery drops over time as its power is drawn, regulators that have smaller dropout voltages tend to provide regulated power over a longer time period than regulators having larger dropout voltages, and, accordingly, using low-dropout voltage regulators in uninterruptible power supplies is desirable.
In a known circuit for a multiple-input, single-output voltage regulator, multiple diodes (one diode for each input of the multiple-input, single-output voltage regulator) and a single-input voltage regulator are arranged so that all of the cathodes of the diodes are connected together and to the input of the single-input voltage regulator. In this circuit, the anode of each diode is connected to the positive terminal of a different power source and the output of the single-input voltage regulator is connected to the load receiving power from the multiple-input, single-output voltage regulator. The diodes in this circuit steer current from the power sources to the input of the single-input voltage regulator such that current from the source with the highest voltage will supply power to the load. The diodes in series with the sources not having the highest voltage will be reversed biased, and, accordingly, will conduct no current.
This approach to providing a multiple-input, single-output voltage regulator is problematic in at least two regards. First, the voltage of the primary power source must always be greater than the voltages of the remaining power sources in order for the primary power source to continue providing current to the load. If at any point, the voltage of any of the remaining power sources exceeds the voltage of the primary power source, the diode associated with the primary power source will be reversed biased and will cease to provide current to the load. Second, the dropout voltage of the multiple-input, single-output voltage regulator is increased by the forward voltage of the diodes forming the inputs of the multiple-input, single-output voltage regulator. This increase in dropout voltage is undesirable because it decreases the effective duration over which a battery providing power to the multiple-input, single-output voltage regulator can do so without the regulator entering dropout.
In another known circuit for a multiple-input, single-output voltage regulator, multiple single-input voltage regulators are arranged in parallel so that the input from each regulator is connected to a different power source and so that the outputs from all of the single-input voltage regulators are connected together and to a load. In this arrangement, the output voltage of each single-input regulator must be set so that the single-input voltage regulator associated with the primary power source has the highest output voltage, the single-input voltage regulator associated with the secondary power source has the second highest output voltage, the single-input regulator associated with the tertiary power source has the third highest output voltage, and so on. By having a higher output voltage than each of the remaining power sources, the single-input voltage regulator associated with the primary power source will cause the outputs of each of the remaining regulators to be pulled above their normal operating points, thus causing them to turn off. However, once the voltage of the primary power source decreases to the point whereat the associated single-input voltage regulator enters dropout, the secondary power source by way of its associated single input voltage regulator will begin providing power to the load. As the voltage of each remaining power source decreases to the point whereat the associated single-input voltage regulator enters dropout, the next remaining power source by way of its associated single-input voltage regulator will begin providing power to the load.
This second approach to providing a multiple-input, single-output voltage regulator is also problematic in at least one regard. Particularly, due to the tolerances of the output voltages of typical single-input voltage regulators, the difference in the output voltages of any two single-output voltage regulators in this approach must be at least twice the output voltage tolerance for any single voltage regulator. This minimum required difference in output voltage causes the voltage output by a multiple-input, single-out voltage regulator implementing this approach to be susceptible to a large voltage drop when transitioning from regulation by one single-input regulator to regulation by another single-input regulator. For example, in a two single-input voltage regulator implementation of this approach, where each regulator has an output voltage tolerance of four percent, the output voltages of the two regulators would have to be separated by at least eight percent. When switching from primary regulation to secondary regulation, and thus from primary power to secondary power, the output voltage of the circuit may drop by up to eight percent. Such a large voltage change may be unacceptable for many loads.